That surprises me. John is somewhat up in age - he's not old at 48 [born 1972, not sure which month] - but he's not young either.
At least...not young enough for something like this to be a complete shocker, as it would be if it happened to a 28-year-old. We can't take our health for granted starting at 40, and we even more cannot take it for granted with each passing decade from there.
John definitely had amazing conditioning. Maybe he ran the cutting agents harder than most? Sheer speculation on my part.
If I recall correctly, John posted his blood work at least once, and all of his numbers were well within the normal range. I suppose, as I have read before, blood test results are only one marker of health, and by no means tell the whole story. That was also several years ago, and who knows if anything changed in his lifestyle or health status since that time.